Developmental space propulsion systems have used macro field effect electrostatic propulsion thrusters that have sub-millimeter to millimeter-sized gaps to create large ion accelerating fields with smaller applied voltages than is possible with conventional ion thrusters, which typically require many thousands of volts. The resulting macro thrusters have a large specific impulse and have a large thrust. However, macro field effect electrostatic propulsion thrusters do require accelerating voltages of many hundreds to a thousand or more volts, which is undesirable. These macro field effect electrostatic propulsion thrusters have been built and tested for use in space but lack possible further reductions in size and weight. These macro field effect electrostatic propulsion thrusters have not been micromachined with micron dimensions, and thus, the size of propulsion arrays using the thrusters will necessarily be large.
In contrast to the macro field effect electrostatic propulsion, micromachined field effect electrostatic propulsion thrusters are in development. The micromachined field effect electrostatic propulsion thrusters are made with accelerating gaps whose dimensions are in micrometer sizes. These micromachined thrusters and arrays made of these micromachined thrusters are the only known space propulsion technology capable of simultaneously providing high specific impulse from 500 to 5,000 seconds, high thrust in micronewtons to Newtons, low specific weight in grams/Newton of thrust, lower specific power demands than conventional electric ion propulsion in N/kW, and much lower required drive voltages in the range of 200-500 V rather than the many thousands of volts for conventional ion thrusters. In addition, these micromachined thrusters have the ability to control the thrust proportionally from nanonewtons to Newtons.
The macromachined field effect electrostatic propulsion thrusters take advantage of micromachined designs using a hollow accelerating electrode with a self-forming cone of liquid metal propellant, such as one made of gallium or indium. The tip radius of this cone forms to radii in the nanometer scale. Thus, the field required to pull ions out of the liquid and accelerate the ions is very low due to a very large field gradient. The accelerating voltage varies both with the desired specific impulse Isp and the material to be ionized. For example, preliminary consideration would indicate that specific impulses of 3,000 ISP could be attained using liquid indium with a potential difference of about 500 volts, and for liquid gallium, the potential difference would be only about 300 volts. These thrusters have high efficiency. This high efficiency, from 500 to 5,000 seconds, indicates that a given thrust should be obtainable with about half the input power than is the case for conventional ion propulsion.
In addition, the micro thruster weight becomes negligible when micromachined on a silicon or other substrate. As an example, initial calculations suggest that a 1 mm by 1 mm array with 105 micron-sized ion sources should produce a thrust level of 7.2 mN at a specific impulse of 1,700 seconds with an ion beam power of only 60 W. Also, because the ionization time constants are extremely short, proportionally variable thrust over a very large thrust range can be achieved by simply pulse-modulating the impressed accelerating voltage.
Prior space thrusters, such as gas thrusters, have been employed in arrays for providing directional microthrust. However, field effect electrostatic propulsion thrusters have not been developed to provide space systems with efficient directional microthrust propulsion and control. Prior space thrusters have required fuel plumbing, valving, pressurants, and external tanks, to perfect the delivery of propellants to the space thrusters. Such fuel plumbing, valving, pressurants, and external tanks in the macroscale for system applications have been unsuitable for use with field effect electrostatic propulsion micromachined thrusters. These and other disadvantages are solved or reduced using the invention.